The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk, and when the disk rotates, air adjacent to the disk moves along with the surface of the disk. The slider flies over the surface of the disk on a cushion of this moving air. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head can include a magnetic write pole and a magnetic return pole, both of which are magnetically connected with one another at a location removed from the air bearing surface, such as by a magnetic back gap layer and a magnetic shaping layer. A non-magnetic, electrically conductive write coil generates a magnetic flux in the write pole and return pole. The write pole has a cross section at the air bearing surface that is much smaller than the cross section of the return pole. The magnetic flux in the return pole and write pole causes a magnetic write field to be emitted to the magnetic medium, thereby recording a magnetic signal thereon. The magnetic flux then flows through the media to return to the return pole wherein it is sufficiently spread out that it does not erase the previously recorded bit.
In recent read head designs, a GMR or TMR sensor has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, or barrier layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned layer and a free layer. First and second leads are connected to the sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
The magnetic read and write heads are very sensitive to any sort of head disk contact. A contact between the head and the disk causes a sever heat spike that can permanently damage the read and write heads. In addition, the contact can cause physical damage to the read or write head or to the disk itself. One way that such a contact can occur is if the disk has a physical asperity. The disk is designed and manufactured to be as close to perfectly smooth and flat as possible. However, in some instances physical asperities can exist, and must be detected on the disk before a finished disk drive product can be assembled and shipped.
One process that has been used to detect such asperities is by the use of optical glide testing. However since there is no mechanical contact involved, such a process cannot really measure the damage potential of a defect such as the hardness of the defect. In addition, this process is limited to a very small spot size of a laser used to perform such a test. As a result throughput using such a process is very low.